Conventional devices for heating and/or frothing a liquid, such as milk, may include a nozzle associated with a source of pressurized steam. When the nozzle is immersed in the liquid, the steam emerging from the nozzle causes both heating and frothing of the liquid. Unfortunately, this process has a number of disadvantages. For example, considerable skill may be required to properly execute this method; e.g., in order to efficiently froth milk, the user must learn how to correctly position the nozzle in a milk container, manually, and continually move the container relative to the nozzle, e.g., in a periodic circular motion. Additionally, it is very difficult to control the heating of the liquid by the steam, and as a result, the frothing efficiency may be reduced and/the temperature of the frothed liquid may not be ideal. Furthermore, steam cooled by the frothed liquid may significantly dilute the frothed liquid with water.
Devices for heating and frothing milk using a stirring or paddling mechanism as a frothing means, instead of steam, have been proposed; however, such devices have not been implemented commercially, e.g., due to inefficiency and/or over-complexity. A device described in U.S. Pat. No. 6,283,625 includes a jug-shaped liquid container associated with a heating unit. The frothing mechanism of this device includes upper and lower paddle groups within the container. During operation of the device, the lower and upper paddle groups may be rotated in opposite directions. This counter rotation mechanism requires a relatively complex transmission mechanism, which may be heavy and difficult to maintain. Additionally, this device may not efficiently froth milk if the device is not filled with an amount of milk suitable for the position of the paddles, and the frothing efficiency is expected to be further reduced during the frothing process as the level of un-frothed milk changes.